A thermocouple microwave power sensor detects the power of a wide-band RF signal. Thin Film and Monolithic thermocouples have been used for power measurements. Thermocouple microwave power sensors suffer from a susceptibility to burn out at higher power levels, parasitic reactances that limit their frequency range, and nonlinear thermocouple response. Typical thermocouple power sensors (shown schematically in FIGS. 1A and 1B) are disclosed by Jackson in Hewlett Packard Journal, September 1974, Vol. 26, No.1, “A Thin-Film/Semiconductor Thermocouple for Microwave Power Measurements”, by Kodato in U.S. Pat. No. 6,518,743, “Wide-band RF signal power detecting element and power detecting device using the same”, and by Delfs, et al. in U.S. Pat. No. 4,789,823, “Power sensor for RF power measurements”.
As shown in FIG. 1A, Jackson uses capacitors to separate the incoming RF (AC) and outgoing measurable (DC) signals. As shown in FIG. 1B, the power dissipating resistor is isolated from the thermocouples, and a capacitor to block incoming DC is optional (not shown). Linearity correction is included in the controls of the power sensor as the accuracy degrades towards the high end of the meter.
In operation, one or more thermocouple pairs measure the temperature difference between two points. The temperature rise is assumed to be proportional to the unknown input power.
In some power sensors, a symmetrical arrangement of power dissipating elements and temperature sensors is used. In this “calorimetric” method, the DC power applied to one of the identical halves of the sensor arrangement is adjusted to match the unknown power applied to the other of the two identical halves of the sensor arrangement. When each of the two temperature sensors reads the same temperature, the known DC power applied is equal to the unknown RF power. A meter employing this method is disclosed by N. R. Erickson in “A fast and sensitive submillimeter waveguide power meter,” in Tenth Int. Space Terahertz Technol. Symp., Charlottesville, Va., March 1999, pp. 501-507.